arXiv subjects

Anne-Marie Lagrange

Publications and source records attributed to Anne-Marie Lagrange.

86 records · Page 5Linked to original sources

arXiv2012

Morphology of the very inclined debris disk around HD 32297

Direct imaging of circumstellar disks at high angular resolution is mandatory to provide morphological information that bring constraints on their properties, in particular the spatial distribution of dust. New techniques combining observing strategy and data processing now allow very high contrast imaging with 8-m class ground-based telescopes (10^-4 to 10^-5 at ~1") and complement space telescopes while improving angular resolution at near infrared wavelengths. We carried out a program at the VLT with NACO to image known debris disks with higher angular resolution in the near IR than ever before in order to study morphological properties and ultimately to detect signpost of planets. The observing method makes use of advanced techniques: Adaptive Optics, Coronagraphy and Differential Imaging, a combination designed to directly image exoplanets with the upcoming generation of "planet finders" like GPI (Gemini Planet Imager) and SPHERE (Spectro-Polarimetric High contrast Exoplanet REsearch). Applied to extended objects like circumstellar disks, the method is still successful but produces significant biases in terms of photometry and morphology. We developed a new model-matching procedure to correct for these biases and hence to bring constraints on the morphology of debris disks. From our program, we present new images of the disk around the star HD 32297 obtained in the H (1.6mic) and Ks (2.2mic) bands with an unprecedented angular resolution (~65 mas). The images show an inclined thin disk detected at separations larger than 0.5-0.6". The modeling stage confirms a very high inclination (i=88°) and the presence of an inner cavity inside r_0~110AU. We also found that the spine (line of maximum intensity along the midplane) of the disk is curved and we attributed this feature to a large anisotropic scattering factor (g~0.5, valid for an non-edge on disk). Abridged ...

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arXiv2011

Searching for faint companions with VLTI/PIONIER. I. Method and first results

Context. A new four-telescope interferometric instrument called PIONIER has recently been installed at VLTI. It provides improved imaging capabilities together with high precision. Aims. We search for low-mass companions around a few bright stars using different strategies, and determine the dynamic range currently reachable with PIONIER. Methods. Our method is based on the closure phase, which is the most robust interferometric quantity when searching for faint companions. We computed the chi^2 goodness of fit for a series of binary star models at different positions and with various flux ratios. The resulting chi^2 cube was used to identify the best-fit binary model and evaluate its significance, or to determine upper limits on the companion flux in case of non detections. Results. No companion is found around Fomalhaut, tau Cet and Regulus. The median upper limits at 3 sigma on the companion flux ratio are respectively of 2.3e-3 (in 4 h), 3.5e-3 (in 3 h) and 5.4e-3 (in 1.5 h) on the search region extending from 5 to 100 mas. Our observations confirm that the previously detected near-infrared excess emissions around Fomalhaut and tau Cet are not related to a low-mass companion, and instead come from an extended source such as an exozodiacal disk. In the case of del Aqr, in 30 min of observation, we obtain the first direct detection of a previously known companion, at an angular distance of about 40 mas and with a flux ratio of 2.05e-2 \pm 0.16e-2. Due to the limited u,v plane coverage, its position can, however, not be unambiguously determined. Conclusions. After only a few months of operation, PIONIER has already achieved one of the best dynamic ranges world-wide for multi-aperture interferometers. A dynamic range up to about 1:500 is demonstrated, but significant improvements are still required to reach the ultimate goal of directly detecting hot giant extrasolar planets.

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arXiv2011

High precision astrometry mission for the detection and characterization of nearby habitable planetary systems with the Nearby Earth Astrometric Telescope (NEAT)

(abridged) A complete census of planetary systems around a volume-limited sample of solar-type stars (FGK dwarfs) in the Solar neighborhood with uniform sensitivity down to Earth-mass planets within their Habitable Zones out to several AUs would be a major milestone in extrasolar planets astrophysics. This fundamental goal can be achieved with a mission concept such as NEAT - the Nearby Earth Astrometric Telescope. NEAT is designed to carry out space-borne extremely-high-precision astrometric measurements sufficient to detect dynamical effects due to orbiting planets of mass even lower than Earth's around the nearest stars. Such a survey mission would provide the actual planetary masses and the full orbital geometry for all the components of the detected planetary systems down to the Earth-mass limit. The NEAT performance limits can be achieved by carrying out differential astrometry between the targets and a set of suitable reference stars in the field. The NEAT instrument design consists of an off-axis parabola single-mirror telescope, a detector with a large field of view made of small movable CCDs located around a fixed central CCD, and an interferometric calibration system originating from metrology fibers located at the primary mirror. The proposed mission architecture relies on the use of two satellites operating at L2 for 5 years, flying in formation and offering a capability of more than 20,000 reconfigurations (alternative option uses deployable boom). The NEAT primary science program will encompass an astrometric survey of our 200 closest F-, G- and K-type stellar neighbors, with an average of 50 visits. The remaining time might be allocated to improve the characterization of the architecture of selected planetary systems around nearby targets of specific interest (low-mass stars, young stars, etc.) discovered by Gaia, ground-based high-precision radial-velocity surveys.

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arXiv2011

Spin-orbit inclinations of the exoplanetary systems HAT-P-8, HAT-P-9, HAT-P-16 and HAT-P-23

We report the measurement of the spin-orbit angle of the extra-solar planets HAT-P-8 b, HAT-P-9 b, HAT-P-16 b and HAT-P-23 b, thanks to spectroscopic observations performed at the Observatoire de Haute-Provence with the SOPHIE spectrograph on the 1.93-m telescope. Radial velocity measurements of the Rossiter-McLaughlin effect show the detection of an apparent prograde, aligned orbit for all systems. The projected spin-orbit angles are found to be lambda=-17 deg (+9.2,-11.5), -16 deg (8), -10 deg (16), +15 deg (22) for HAT-P-8, HAT-P-9, HAT-P-16 and HAT-P-23 respectively, with corresponding projected rotational velocities of 14.5 (0.8), 12.5 (1.8), 3.9 (0.8), and 7.8 (1.6) km/s. These new results increase to 37 the number of accurately measured spin-orbit angles in transiting extrasolar systems. We conclude by drawing a tentative picture of the global behaviour of orbital alignement, involving the complexity and diversity of possible mechanisms.

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arXiv2010

SOPHIE velocimetry of Kepler transit candidates. I. Detection of the low-mass white dwarf KOI-74b

The Kepler mission has detected transits and occultations of a hot compact object around an early-type star, the Kepler Object of Interest KOI 74. The mass of this transiting object was photometrically assessed in a previous study using the presence of the relativistic beaming effect (so-called `Doppler boosting') in the light curve. Our aim was to provide a spectroscopic validation of this pioneering approach. We measured the radial velocity variations of the A1V star KOI 74 with the SOPHIE spectrograph at the 1.93-m telescope of the Observatoire de Haute-Provence (France). Radial velocity measurements of this star are challenging because of the high level of stellar pulsations and the few available spectral lines. Using a technique dedicated to early-type main-sequence stars, we measured radial velocity variations compatible with a companion of mass 0.252+/-0.025 Msun, in good agreement with the value derived from the Kepler light curve. This work strengthens the scenario suggesting that KOI 74 is a blue straggler orbited by a stellar core despoiled of its envelope, the low-mass white dwarf KOI 74b.

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arXiv2010

First Results From VLT NACO Apodizing Phase Plate: 4-micron Images of the Exoplanet beta Pictoris b

Direct imaging of exoplanets requires both high contrast and high spatial resolution. Here, we present the first scientific results obtained with the newly commissioned Apodizing Phase Plate coronagraph (APP) on VLT/NACO. We detected the exoplanet beta Pictoris b in the narrow band filter centered at 4.05 micron (NB4.05). The position angle (209.13 +- 2.12 deg) and the projected separation to its host star (0."354 +- 0".012, i.e., 6.8 +- 0.2 AU at a distance of 19.3 pc) are in good agreement with the recently presented data from Lagrange et al. (2010). Comparing the observed NB4.05 magnitude of 11.20 +- 0.23 mag to theoretical atmospheric models we find a best fit with a 7-10 M_Jupiter object for an age of 12 Myr, again in agreement with previous estimates. Combining our results with published L' photometry we can compare the planet's [L' - NB4.05] color to that of cool field dwarfs of higher surface gravity suggesting an effective temperature of ~1700 K. The best fit theoretical model predicts an effective temperature of ~1470 K, but this difference is not significant given our photometric uncertainties. Our results demonstrate the potential of NACO/APP for future planet searches and provides independent confirmation as well as complementary data for beta Pictoris b.

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arXiv2010

The Earth as an extrasolar transiting planet: Earth's atmospheric composition and thickness revealed by Lunar eclipse observations

An important goal within the quest for detecting an Earth-like extrasolar planet, will be to identify atmospheric gaseous bio-signatures. Observations of the light transmitted through the Earth's atmosphere, as for an extrasolar planet, will be the first step for future comparisons. We have completed observations of the Earth during a Lunar eclipse, a unique situation similar to that of a transiting planet. We aim at showing what species could be detected in its atmosphere at optical wavelengths, where a lot of photons are available in the masked stellar light. We present observations of the 2008 August 16 Moon eclipse performed with the SOPHIE spectrograph at the Observatoire de Haute-Provence. Locating the spectrograph fibers in the penumbra of the eclipse, the Moon irradiance is then a mix of direct, unabsorbed Sun light and solar light that has passed through the Earth's limb. This mixture essentially reproduces what is recorded during the transit of an extrasolar planet. We report here the clear detection of several Earth atmospheric compounds in the transmission spectra, such as ozone, molecular oxygen, and neutral sodium as well as molecular nitrogen and oxygen through the Rayleigh signature. Moreover, we present a method that allows us to derive the thickness of the atmosphere versus the wavelength for penumbra eclipse observations. We quantitatively evaluate the altitude at which the atmosphere becomes transparent for important species like molecular oxygen and ozone, two species thought to be tightly linked to the presence of life. The molecular detections presented here are an encouraging first attempt, necessary to better prepare for the future of extremely-large telescopes and transiting Earth-like planets. Instruments like SOPHIE will be mandatory when characterizing the atmospheres of transiting Earth-like planets from the ground and searching for bio-marker signatures.

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arXiv2010

Deep infrared imaging of close companions to austral A- and F-type stars

The search for substellar companions around stars with different masses along the main sequence is critical to understand the different processes leading to the formation of low-mass stars, brown dwarfs, and planets. In particular, the existence of a large population of low-mass stars and brown dwarfs physically bound to early-type main-sequence stars could imply that the massive planets recently imaged at wide separations (10-100 AU) around A-type stars are disc-born objects in the low-mass tail of the binary distribution. Our aim is to characterize the environment of early-type main-sequence stars by detecting brown dwarf or low-mass star companions between 10 and 500 AU. High contrast and high angular resolution near-infrared images of a sample of 38 southern A- and F-type stars have been obtained between 2005 and 2009 with the instruments VLT/NaCo and CFHT/PUEO. Multi-epoch observations were performed to discriminate comoving companions from background contaminants. About 41 companion candidates were imaged around 23 stars. Follow-up observations for 83% of these stars allowed us to identify a large number of background contaminants. We report the detection of 7 low-mass stars with masses between 0.1 and 0.8 Msun in 6 multiple systems: the discovery of a M2 companion around the A5V star HD14943 and the detection of the B component of the F4V star HD41742 quadruple system; we resolve the known companion of the F6.5V star HD49095 as a short-period binary system composed by 2 M/L dwarfs. We also resolve the companions to the astrometric binaries iot Crt (F6.5V) and 26 Oph (F3V), and identify a M3/M4 companion to the F4V star omi Gru, associated with a X-ray source. The global multiplicity fraction measured in our sample of A and F stars is >16%. A parallel velocimetric survey of our stars let us conclude that the imaged companions can impact on the observed radial velocity measurements.

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arXiv2010

Reconstructing the solar integrated radial velocity using MDI/SOHO

Searches for exoplanets with radial velocity techniques are increasingly sensitive to stellar activity. It is therefore crucial to characterize how this activity influences radial velocity measurements in their study of the detectability of planets in these conditions. In a previous work we simulated the impact of spots and plages on the radial velocity of the Sun. Our objective is to compare this simulation with the observed radial velocity of the Sun for the same period. We use Dopplergrams and magnetograms obtained by MDI/SOHO over one solar cycle to reconstruct the solar integrated radial velocity in the Ni line 6768 Å. We also characterize the relation between the velocity and the local magnetic field to interpret our results. We obtain a stronger redshift in places where the local magnetic field is larger (and as a consequence for larger magnetic structures): hence we find a higher attenuation of the convective blueshift in plages than in the network. Our results are compatible with an attenuation of this blueshift by about 50% when averaged over plages and network. We obtain an integrated radial velocity with an amplitude over the solar cycle of about 8 m/s, with small-scale variations similar to the results of the simulation, once they are scaled to the Ni line. The observed solar integrated radial velocity agrees with the result of the simulation made in our previous work within 30%, which validates this simulation. The observed amplitude confirms that the impact of the convective blueshift attenuation in magnetic regions will be critical to detect Earth-mass planets in the habitable zone around solar-like stars.

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arXiv2009

Constraining the orbit of the possible companion to Beta Pictoris: New deep imaging observations

We recently reported on the detection of a possible planetary-mass companion to Beta Pictoris at a projected separation of 8 AU from the star, using data taken in November 2003 with NaCo, the adaptive-optics system installed on the Very Large Telescope UT4. Eventhough no second epoch detection was available, there are strong arguments to favor a gravitationally bound companion rather than a background object. If confirmed and located at a physical separation of 8 AU, this young, hot (~1500 K), massive Jovian companion (~8 Mjup) would be the closest planet to its star ever imaged, could be formed via core-accretion, and could explain the main morphological and dynamical properties of the dust disk. Our goal was to return to Beta Pic five years later to obtain a second-epoch observation of the companion or, in case of a non-detection, constrain its orbit. Deep adaptive-optics L'-band direct images of Beta Pic and Ks-band Four-Quadrant-Phase-Mask (4QPM) coronagraphic images were recorded with NaCo in January and February 2009. We also use 4QPM data taken in November 2004. No point-like signal with the brightness of the companion candidate (apparent magnitudes L'=11.2 or Ks ~ 12.5) is detected at projected distances down to 6.5 AU from the star in the 2009 data. As expected, the non-detection does not allow to rule out a background object; however, we show that it is consistent with the orbital motion of a bound companion that got closer to the star since first observed in 2003 and that is just emerging from behind the star at the present epoch. We place strong constraints on the possible orbits of the companion and discuss future observing prospects.

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arXiv2007

Search for exoplanets with the radial-velocity technique: quantitative diagnostics of stellar activity

Aims: Stellar activity may complicate the analysis of high-precision radial-velocity spectroscopic data when looking for exoplanets signatures. We aim at quantifying the impact of stellar spots on stars with various spectral types and rotational velocities and comparing the simulations with data obtained with the HARPS spectrograph. Methods: We have developed detailed simulations of stellar spots and estimated their effects on a number of observables commonly used in the analysis of radial-velocity data when looking for extrasolar planets, such as radial-velocity curves, cross-correlation functions, bisector spans and photometric curves. The computed stellar spectra are then analyzed in the same way as when searching for exoplanets. Results: 1) A first grid of simulation results is built for F-K type stars, with different stellar and spot properties. 2) It is shown quantitatively that star spots with typical sizes of 1% can mimic both radial-velocity curves and the bisector behavior of short-period giant planets around G-K type stars with a vsini lower than the spectrograph resolution. For stars with intermediate vsini, smaller spots may produce similar features. In these cases, additional observables (e.g., photometry, spectroscopic diagnostics) are mandatory to confirm the presence of short-period planets. We show that, in some cases, photometric variations may not be enough to clearly rule out spots as explanations of the observed radial-velocity variations. This is particularly important when searching for super-Earth planets. 3) It is also stressed that quantitative values obtained for radial-velocity and bisector span amplitudes depend strongly on the detailed star properties, on the spectrograph used, on the set of lines used, and on the way they are measured.

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arXiv2005

The Arches Cluster - Evidence for a Truncated Mass Function?

We have analyzed high-resolution, adaptive optics (AO) HK observations of the Arches cluster obtained with NAOS/CONICA. With a spatial resolution of 84 mas, the cluster center is uniquely resolved. From these data the present-day mass function (MF) of Arches is derived down to about 4 Msun. The integrated MF as well as the core and 2nd annulus MFs are consistent with a turn-over at 6-7 Msun. This turn-over indicates severe depletion of intermediate and low-mass stars in the Arches cluster, possibly caused by its evolution in the Galactic Center environment. The Arches MF represents the first resolved observation of a starburst cluster exhibiting a low-mass truncated MF. This finding has severe implications for stellar population synthesis modelling of extragalactic starbursts, the derivation of integrated properties such as the total mass of star clusters in dense environments, the survival of low-mass remnants from starburst populations, and chemical enrichment during starburst phases.

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arXiv2003

Hot Very Small dust Grains in NGC 1068 seen in jet induced structures thanks to VLT/NACO adaptive optics

We present K, L and M diffraction-limited images of NGC 1068 obtained with NAOS+CONICA at VLT/YEPUN over a 3.5" field around the central engine. Hot dust (Tcol = 550-650 K) is found in three different regions : (a) in the true nucleus, seen as a slightly NS elongated, core of extremely hot dust, "resolved" in K and L with respective diameters of ~5 pc and 8.5 pc ; (b) along the NS direction, as a "spiral arm" and a southern tongue ; (c) as a set of parallel elongated nodules ("wave-like") bracketting the jet. Several structures observed on radio maps, mid-IR or HST UV-visible maps are seen, so that a precise registration can be done from UV to 6 cm. These results do support the current interpretion that source (a) corresponds to emission from dust near sublimation temperature delimiting the walls of the cavity in the central obscuring torus. Structure (b) is thought to be a mixture of hot dust and active star forming regions along a micro spiral structure that could trace the tidal mechanism bringing matter to the central engine. Structure c)which was not known, exhibits too high a temperature for "classical'' grains ; it is most probably the signature of transiently heated very small dust grains (VSG) : "nano-diamonds", which are resistant and can form in strong UV field or in shocks, are very attractive candidates. The "waves'' can be condensations triggered by jet induced shocks, as predicted by recent models. First estimates, based on a simple VSG model and on a detailed radiative transfer model, do agree with those interpretations, both qualitatively and quantitatively.

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arXiv2003

VLT/NACO infrared adaptive optics images of small scale structures in OMC1

Near-infrared observations of line emission from excited H2 and in the continuum are reported in the direction of the Orion molecular cloud OMC1, using the European Southern Observatory Very Large Telescope UT4, equipped with the NAOS adaptive optics system and the CONICA infrared array camera. Spatial resolution has been achieved at close to the diffraction limit of the telescope (0.08" - 0.12") and images show a wealth of morphological detail. Structure is not fractal but shows two preferred scale sizes of 2.4" (1100 AU) and 1.2" (540 AU), where the larger scale may be associated with star formation.

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